It is well known, since its development by Benjamin et al. in U.S. Pat. No. 3,591,362, that the basic mechanism of mechanical milling, alloying or grinding, consists of a repeated deformation, fracture and cold welding by high energetic ball collisions. Depending on the dominant process during milling, such as fracturing, welding or micro-forging, a particle may become smaller through fracturing or may grow through agglomeration. This process has been extensively used for particle size reduction or growth, shape change, solid-state alloying or blending, modifying, changing or altering properties of material like the density or flowability, mixing or blending of two or more materials and agglomeration. The following patents are examples of issued patents which disclose methods of producing mechanically alloyed composite powders and consolidated products: U.S. Pat. Nos. 3,591,362; 3,660,049; 3,723,092; 3,728,088; 3,738,817; 3,740,210; 3,785,801; 3,809,549; 3,737,300; 3,746,581; 3,749,612; 3,816,080; 3,844,847; 3,865,572; 3,814,635; 3,830,435; 3,877,930; 3,912,552; 3,926,568; and 4,134,852.
Ball-milling has also been used to produce nanocrystalline materials with a structure having a large number of grains and grain boundaries, within particles globally in the micrometer scale size. In the literature, there are a large number of scientific papers attesting the capability of this technique to do so. Nanostructured materials can also be obtained by methods that do not involve ball-milling. These methods include DC and RF plasma processes, vapor-phase reactions, sol-gel techniques, combustion, emulsion and laser or hydrothermal synthesis. All these techniques produce mainly nanostructured materials with nanometer sized particles, bonded together by Van der Waal's electrostatic forces (a soft agglomeration) and present a very low density.
One of the primary limitations with nanoparticles is that nanoscale particles cannot be processed directly in many applications such as thermal spraying because of the extremely low density and flowability. Thus, there is a vital need for densifying and consolidating by agglomeration such particles into micrometer scale to enable processing. Usually, agglomeration of powders is achieved via spray drying, fluid bed agglomeration, or granulation. As a drawback, these processes often require an organic binder to ensure particle adherence, which may remain after agglomeration. There is also a risk for powder to react with the agglomeration media to form undesirable species such as oxides.
Ball milling has been used to produce powders for different applications. However, it is believed that the use of ball milling to agglomerate nano-sized particles into micrometric-sized particles has not been previously proposed. This is also believed for association of two different techniques, namely, production of nanostructured powders of a nanoscale particle size, and agglomeration of the nanoscale powders into nanostructured micrometric powders with a relatively high density.
In U.S. Pat. No. 5,631,044, issued to Rangaswamy et al., there is disclosed an improved method utilizing a high energy ball mill for preparing binder-free clad powders. The powders are indicated to be useful as thermal spray powders. It is stated that the binder-free clad powders have a core material with a particle size range of about 10 to 200 microns. These are coated or partially coated with a second material, significantly smaller than the particle size of the core material with a particle size ranging from 0.1 to 20 microns. At least one of the two materials must be deformable within the high energy ball mill. The processing time is less than about one hour. The result is a thermal spray powder having an average particle size from about 10 to 150 microns.
In U.S. Pat. No. 4,749,545, issued to Begg et al., a method is discussed which allows preparation by ball milling of composite particles made of hard material such as SiC and a matrix of aluminum or magnesium. The former material has a particle size less than 50 microns while the second is less than 100 microns. The method enables mixing of a high proportion of hard materials to be incorporated into the composite.
Similar to the previous patent, in U.S. Pat. No. 4,818,567, issued to Kemp, Jr. et al., metallic coated particles are disclosed. The particles have a core consisting essentially of metals, metal alloys, ceramics, ceramic glasses, and a coating relatively uniformly distributed on the core. The coating comprises a relatively ductile and/or malleable metallic material selected from metals and metal alloys.
In U.S. Pat. No. 4,787,561, issued to Kemp, Jr. et al., a process involving the grinding of particles of a ductile and/or malleable material which may be metal, metal alloy, or metal-ceramic composites, with a combination of parameters and environment in a ball attritor mill was disclosed. The product is densely packed particles, having a substantially granular appearance and an aspect ratio of from greater than 1 to about 50 and a mean particle size of less than about 20 micrometers in diameter.
All the above-mentioned patents have used ball milling as a tool to coat a core material or to prepare composite materials that have a microcrystalline structure. None of these reports the use of ball milling to agglomerate and consolidate nanoparticle powders.